1. Field of the Invention
The present invention relates to an optical transmission system, and more particularly to a wideband optical fiber amplifier which amplifies both C-band and L-band optical signals for an optical transmission system.
2. Description of the Related Art
With the amount of data growing at an explosive rate recently, there is a large demand to broaden the transmission bandwidth of Wavelength Division Multiplexing (WDM) optical transmission systems. Accordingly, research is now active in a wideband transmission system using both C-band having a wavelength in the range of 1530 nm to 1560 nm and L-band having a wavelength in the range of 1568 nm to 1610 nm. In optical transmission systems, an erbium-doped fiber amplifier (EDFA) (a fiber amplifier doped with the rare-earth element erbium) is generally used as an optical fiber amplifier for amplifying optical signals. This amplifier works only over a bandwidth of about 30 nm in the C-band and the L-band. Although a Raman fiber amplifier (RFA) has a sufficiently broad bandwidth to amplify both C-band and L-band optical signals, it requires a high pump power to obtain a desired gain. Therefore, an EDFA is more generally used as a wideband optical fiber amplifier. However, most EDFAs have a parallel structure for separately amplifying C-band optical signals and L-band optical signals.
FIG. 1 shows a conventional wideband optical fiber amplifier. Optical fiber amplifier 100 is connected to an external optical fiber 110 and comprises first and second amplifying sections 160 and 170. The optical fiber amplifier 100 also includes first and second wavelength selective couplers (WSCs) 121 and 122 for connecting first and second amplifying sections 160 and 170 in a parallel structure.
First wavelength selective coupler 121 divides optical signals having wavelengths of 1550 to 1590 nm, which are inputted through external optical fiber 110, into 1550 nm wavelength band (C-band) signals and 1590 nm wavelength band (L-band) signals, and outputs C-band optical signals to first amplifying section 160 and L-band optical signals to second amplifying section 170.
First amplifying section 160 includes first and second optical isolators (ISOs) 131 and 132, first and second pump laser diodes (LDs) 141 and 142, third and fourth wavelength selective couplers 123 and 124, and a first erbium-doped optical fiber 151. First and second optical isolators 131 and 132 block a light emitted in the backward direction, such as a noise of amplified spontaneous emission (ASE) from first erbium-doped optical fiber 151 and a reflected light. First pump laser diode 141 outputs a first pump light having a 980 nm wavelength. Third wavelength selective coupler 123 outputs the first pump light and the C-band optical signals to first erbium-doped optical fiber 151. Second pump laser diode 142 outputs a second pump light having a 1480 nm wavelength. Fourth wavelength selective coupler 124 outputs the second pump light to first erbium-doped optical fiber 151 and passes the amplified C-band optical signals. First erbium-doped optical fiber 151 is pumped in both directions by the first and second pump lights, thereby amplifying and outputting the inputted C-band optical signals.
Second amplifying section 170 includes third and fourth optical isolators 133 and 134, third and fourth pump laser diodes 143 and 144, fifth and sixth wavelength selective couplers 125 and 126, and a second erbium-doped optical fiber 152. Third and fourth optical isolators 133 and 134 block light emitted in the backward direction, such as an ASE noise outputted from second erbium-doped optical fiber 152 and a reflected light. Third pump laser diode 143 outputs a third pump light having a 980 nm wavelength. Fifth wavelength selective coupler 125 outputs the third pump light and the L-band optical signals to second erbium-doped optical fiber 152. Fourth pump laser diode 144 outputs a fourth pump light having a 1480 nm wavelength. Sixth wavelength selective coupler 126 outputs the fourth pump light to second erbium-doped optical fiber 152 and passes the amplified L-band optical signals. Second erbium-doped optical fiber 152 is pumped in both directions by the third and fourth pump lights, thereby amplifying and outputting the inputted L-band optical signals.
Second wavelength selective coupler 122 couples the C-band optical signals and L-band optical signals received from first and second amplifying sections 160 and 170, respectively, and outputs the coupled signals through external optical fiber 110.
In conventional wideband optical fiber amplifiers as explained above, the second amplifying section for amplifying L-band optical signals has a low amplification efficiency and thus requires the second erbium-doped optical fiber to be long. Also, the second amplifying section requires a higher pump power and has a high noise factor in the L-band. In addition, conventional wideband optical fiber amplifiers require a complicated electric control circuit to control amplification gain, because it uses a large number of pump laser diodes.